U.S. pipeline gas is generally very lean, with heating values ranging from 1000 to 1070 Btu/scf, and more recently, FERC (the Federal Energy Regulatory Commission) has established guidelines and specifications for natural gas import. These guidelines require the Wobbe Index of the import gas to be within +/−4% with respect to the local gas quality, with a maximum value of 1400. California, which traditionally uses a very lean gas, requires a significantly lower Wobbe Index for the import gas. For example, the local air emission agency SCAQMD (Southern California Air Quality Management District) specifies a maximum Wobbe Index of 1360.
Unfortunately, the heating value of unprocessed import LNG is often significantly higher due to the relatively high ethane and propane content, which is not only incompatible with local rules and regulations, but also incompatible with many residential, commercial, and industrial burners. Examples for the wide variations in LNG composition, heating value, and Wobbe Index for LNG export terminals in the Atlantic, Pacific basins and the Middle East are shown in FIG. 1. As can be seen from FIG. 1, only Alaska's LNG can meet the gas quality specifications without nitrogen dilution for import to California while the remaining LNG requires nitrogen blending and/or NGL (natural gas liquids) extraction.
FIG. 2 illustrates the reduction in Wobbe Index before and after nitrogen dilution for the various LNG sources, up to a maximum 3 mol% N2 limit. As can be taken from FIG. 2, less than half of the LNG sources meet the California Wobbe Index even with maximum nitrogen dilution. Moreover, due to the relatively tight margins on meeting the California Wobbe Index specification, changes in Wobbe Index due to weathering in the LNG storage may result in off specification product. The weathering effect of LNG from natural boil-off from the storage tanks enriches LNG in heavier components (i.e., C2+) over time, eventually rendering the weathered gas unacceptable as a pipeline gas with a higher Wobbe Index. While the weathering effect typically increases the Wobbe Index by a relatively small amount (e.g., about 3 to 6 points), such increase is problematic for marginal LNGs.
In various presently known LNG processing configurations to meet the Wobbe Index, non-methane components are removed from the LNG in a process that vaporizes the LNG in a demethanizer using a reboiler and re-condenses the demethanizer overhead to the sendout liquid that is then pumped and vaporized (see e.g., U.S. Pat. No. 6,564,579). While such configurations and methods typically operate satisfactorily for heating value or Wobbe Index control, they will require markets for the extracted NGL products, which are not always available. Moreover, in most cases where LNG terminals are configured for BTU delivery to commercial and residential users, there are no economic incentives for NGL extraction.
Alternatively, anti-weathering configurations can be implemented to reduce increase of Wobbe index as described in U.S. Pat. No. 7,201,002. Here the boil-off vapor is condensed within the confines of the tank using LNG refrigeration and pressure regulation. Similarly, as shown in U.S. Pat. No. 6,530,241, boil-off vapors can be reliquefied on board to control Wobbe index and product loss. However, such configurations are typically limited to either on-board systems that are inflexible with respect to changing and relatively large vapor loads, and/or will require cryogenic equipment and relatively large capital cost. Other systems and methods with similar difficulties are described in U.S. Pat. Nos. 3,894,856 and 4,675,037, U.S. Pat. App. No. 2008/0308175, and WO 2005/047761.
Therefore, while various LNG heating value control methods are known in the art, all or almost all of them suffer from one or more disadvantages, especially where import LNG is used, where NGL markets do not exist, and where the Wobbe Index of the import LNG only marginally meets local specifications. Thus, there is still a need for improved configurations and methods for maintaining the Wobbe Index while providing operating flexibility for the LNG regasification terminals with lower energy consumption.